Findings:

Figure on left is a contour plot of DOC in the surface 300 m at BATS. Note the build up of DOC in surface waters after season stratification, export during mixing then re-mineralization at depth after stratification (Hansell and Carlson 2001). Figure on right is a contour plot of prokaryotic cell abundance at BATS, showing elevated concentrations of prokaryotic biomass below the euphotic zone. The elevated biomass persists for several weeks and coincides with the periodic drawdown of DOC in the upper mesopelagic layer. Graphs provided by C. Carlson (Oceanic Microbial Observatory)

The northwestern Sargasso Sea displays seasonal dynamics in DOC accumulation and distribution of biomass as well prokaryotic community structure. At BATS DOC accumulates rapidly within the euphotic zone shortly after restratification of the water column, and remains at elevated concentrations in the surface waters through the summer and into early fall. Winter convective overturn can result in a portion of the accumulated DOC being trapped between 140-300 m. The removal of the DOC from this zone is on the time scale of weeks. Corresponding microbial biomass is also elevated below the euphotic zone following the seasonal deep convective mixing.

Above is an example of temporal and spatial dynamics of bacterioplankton groups in the upper 300 m of BATS in the year 2003. See contour plots in alternative format. Materials provided by C. Carlson (Oceanic Microbial Observatory)

Temporal increases in bacterioplankton abundance in the upper mesopelagic have been linked to spatial and temporal patterns of Dissolved Organic Carbon (DOC) dynamics at the BATS site. Here we use Terminal Restriction Fragment Length Polymorphism (T-RFLP), clone library, phylogenetic, and bulk nucleic acid hybridization analyses to identify, characterize, and quantify spatial and temporal patterns in marine bacterioplankton communities. Nonmetric multidimensional scaling of bacterial 16S rDNA terminal restriction fragments from monthly surface and 200 m seawater samples demonstrated repeatable temporal trends in bacterial community structure within the different depth horizons. SAR11, marine Actinobacteria, and OCS116-related fragments increased following convective overturn >200 m. Quantitative hybridizations provided additional data supporting spatial and temporal patterns of distribution and abundance for the SAR11, SAR202, and marine Actinobacteria clusters. Increases in the abundance of the SAR11, marine Actinobacteria, and OCS116-related fragments following deep convective mixing events (>200m) at BATS suggests that representatives of these groups may play important roles in DOC dynamics at BATS.

Learn more about this relationship by reading- Carlson, C.A., S.J. Giovannoni, D.A. Hansell, S.J. Goldberg, R. Parsons, and K. Vergin. 2004. Interactions between DOC, microbial processes, and community structure in the mesopelagic zone of the northwestern Sargasso Sea. Limnology and Oceanography 49: 1073-1083.

Clearly, picophytoplankton, which are photosynthetic, would be expected to be more dominant at the surface where sunlight is readily available. Roseobacter, while not photosynthetic itself, has been closely linked to photosynthetic algae by an ability to metabolize dimethylsulfonio-propionate (DMSP), a compound abundantly produced by marine algae. This metabolic capability would explain why Roseobacter is also more prevalent in the surface waters at BATS. Until recently, it was less clear what adaptations were possessed by SAR11a that allow it also to dominate at the surface. Sequencing of a coastal variant of the SAR11a clade has revealed the presence of a gene coding for proteorhodopsin. This protein uses light to drive the export of protons and the energy of the re-entry of protons can be harnessed to convert ADP to ATP. Still, it is unknown why these two groups are alternately dominant in the surface waters suggesting that other environmental factors other than light are important.

Copyright on all images and material by Craig Carlson and Stephen Giovannoni 2005.